Process for encapsulating transistors



June 29, 1965 E. M MATEA l 3,191,268

PROCESS FOR ENCAPSULATING TRANSISTORS Original Filed Feb. 28, 1958 nlllllllll nfronusv 5 United States Patent O 3,191,268 PRGCESS FOR ENCAPSULATENG TRANSISTRS Edmund M. Matea, Covina, Calif., assigner to General Motors Corporation, Detroit, Mich., a corporation of Delaware Original application Feb. 28, 1958, Ser. No. 71,275, now Patent No. 3,113,252, dated Dec. 3, 1963. Divided and this application Jan. 16, 1963, Ser. No. 251,803 3 Claims. @L29-155.5)

This invention relates to a method of encapsulation for electronic devices.

This patent application is a division of mycopending United States patent application Serial No. 718,275 Matea, which was tiled February 28, 1958, now Patent No. 3,113,252 granted December 3, 1963.

Transistors are conventionally hermetically sealed within an enclosure to protect them during normal use. The leads to the various electrodes are either brought through insulators in the housing or may form a part of the housing itself. As an example, in one type of transistor assembly, the base upon which the transistor is mounted is soldered to the collector electrode and forms an electrical lead for the same. The base also forms `a mechanical closure member for the end of the cap which surrounds the transistor. Previously, caps were sealed to the base members by soldering or welding.

However, a transistor, both the main body, the ohmic and rectifying electrodes and soldered leads, are all affected by even moderate temperatures. To apply sutilcient heat to the periphery of the cap to weld or solder it to the base, often made the transistor unfit for use, since an excess amount of heat, through inadvertence, might reach a vital spot.

It is therefore an object in making this invention to provide a method of sealing a semiconductive body within an enclosure without the use of heat.

It is a further object in making this invention to provide a cold hermetic sealing process for fabricating a transistor.

With these and other objects in view which will become apparent as the specification proceeds, the invention will be best understood by reference to the following specilication and claims and the illustration in the accompanying drawings in which:

FIGURE l is a vertical section taken through a transistor assembly showing the cap in place on the base but no sealing step having been performed;

FIGURE 2 is a perspective view showing the cap and base in spaced relation;

FIGURE 3 is a vertical sectional view of the transistor assembly and a partial sectional view of the forming die for crimping over a portion of the base to clamp the cap;

FIGURE 4 is a view similar to FIGURE 3 showing the die forced home to clamp the edge of the cap;

FIGURE 5 is a view similar to FIGURE 4 with the die removed; and

FIGURE 6 is an enlarged sectional view through the rim of the cap and base showing the clamping action of the crimped-over portion.

Referring now more specifically to the drawings, the two parts of the transistor which -i't is desired to secure together `and seal are best shown in FIGURE 2. This includes a cylindrical cap 2 having .a flat flanged edge d at right angles to the main axis of the cap and extending out for a short distance therefrom. The base 6 of metal having good electrical and heat conducting properties has a centrally located pedestal S upon which the transistor body 10 is adapted to be mounted with the collector electrode 12 being soldered to the top of the pedestal S. This, of course, mechanically connects the transistor to the mounting base. Various other electrodes 13 and 15 `are connected through leads 1d and 16 respectively which exice tend through the mounting base and are electrically insulated therefrom by glass eyes 1S.

Around the area upon which the transistor is mounted and through which the leads pass, there is cut out an annular groove 19 having some depth. The radial dimension of the groove is approximately equal to the width of the flange 4 and is so cut that the flange of the cap may fall into the groove when placed over the transistor assembly. FIGURE 1 therefore shows the assembly in place with the cap resting on the bottom of the groove 19. A die Ztl formed as shown in FIGURE 3 with a central aperture 22 lits down over the cap 2 and has an irregularly shaped annular cutting section 24. This section includes a Asharpened annular ridge 26 whose diameter is slightly greater than that of the groove 19 and is provided to peel olf or pare a section of the base and fold it down over the outwardly projecting edge of the flange 4. It therefore has a tapered inner surface 2S which as the die is brought down and pressed into the base, folds that portion 30 over against the flange 4. FIGURE 4 shows the assembly with the die brought home and crimping a portion of the base over the toe of the flange 4. FIGURE 5 shows the final assembly with the die Ztl removed. FIGURE 6 shows in enlarged section, how this material is deformed to provide a satisfactory crimping and seal.

In order to assure that the two pieces will be so closely associated as to form a hermetic seal, with this form of assembly, there is introduced into the groove 19 prior to :assembly a polymer fiuorocarbon compound known commercially as Kel-F. This fluorocarbon is mixed with a small amount of trichlorethylene. While various proportions of the two compounds may operate satisfactorily, it has been found that three grams of Kel-F mixed with mils of trichlorethylcne form a satisfactory compound. In order to introduce this .compound into the bott-om of the groove, the cap 2 is dipped into the solution, lifted out, and placed in a drying atmosphere for some time to dry out the trichlorethylene. It has been found that placing it in an oven at C. for 10 minutes is sufficient drying. rfhe parts are then assembled and the die applied. As an alternate method the groove 19 in the base is filled with the solution and dried in the same manner. The two parts are then assembled as shown in FIG- URE l and mechanically sealed.

This method provides a very satisfactory encapsulation method for hermetically sealing transistor assemblies.

I claim:

1. The method of making an encapsulated signal translating device which comprises the steps of supporting a signal translating body on a metallic capsule base member, said base member having a relieved portion surrounding said body for mating engagement with a radially extending perimetric flange on a capsule cover member, coating at least one mating face of said members with a fiuorocarbon sealing material, placing said cover .over said signal translating body in said rel-ieved area on said base, cutting a portion of said base member adjacent said relieved area, folding down the cut portion onto said cover flange and pressing the folded base member portion against said cover flange.

2. The method of making an encapsulated transistor ywhich comprises the steps of supporting a transistor body on a metallic capsule base member, said base member having a groove surrounding said body for mating engagement with a radially extending perimetric flange on a capsule cover member, coating at least one mating face of said members with a fluorocarbon sealing material, placing said cover over said signal translating body in said groove on said base, cutting a portion of said base member adjacent said groove, folding down the cut portion onto said cover flange and pressing the folded base member portion against said cover flange.

3. The process as described in claim 2 wherein the 2,724,893 11/55 OConnor etal. 220-81 uorocarbon sealing material is applied to the groove in 2,745,044 5/56 Lingel 317-234 lthe base member yand the sealing material is .a mixture of 2,957,236 r10/60 Kreuchen 29-470.1 a fluoroearbonY sealant and a Ithinning Iagent that is dried 3,024,299 3/62,` Nijhuis et al. 29-4701 X before the cover is secured to the base member. n 5 Y OTHER, REFERENCES Y References Cited by the Examiner The Condensed Chemical Dictionary (6th edition), UNITED STATES 'PATENTS' published by Reinhold Publishing Corporation (1961) v a e 639 relied u on 2,220,902 11/40 Hastings et al. (p g p 2,331,449 10/43 Wittenberg 29-511 X 19 JOHN F. CAMPBELL, Primary Examiner. 

1. THE METHOD OF MAKING AN ENCAPSULATED SIGNAL TRANSLATING DEVICE WHICH COMPRISES THE STEPS OF SUPPORTING A SIGNAL TRANSLATING BODY ON A METALLIC CAPSULE BASE MEMBER, SAID BASE MEMBER HAVING A RELIEVED PORTION SURROUNDING SAID BODY FOR MATING ENGAGEMENT WITH A RADIALLY EXTENDING PERIMETRIC FLANGE ON A CAPSULE COVER MEMBER, COATING AT LEAST ONE MATING FACE OF SAID MEMBERS WITH A FLUOROCARBON SEALING MATERIAL, PLACING SAID COVER OVER SAID SIGNAL TRANSLATING BODY IN SAID RELIEVED AREA ON SAID BASE, CUTTING A PORTION OF SAID BASE MEMBER ADJACENT SAID RELIEVED AREA, FOLDING DOWN THE CUT PORTION ONTO SAID COVER FLANGE AND PRESSING THE FOLDED BASE MEMBER PORTION AGAINST SAID COVER FLANGE. 